There exists various known load detection devices for a vehicle seat for detecting a load applied by an occupant, to be adapted to a vehicle having an airbag for protecting an occupant seated on the vehicle seat, in order to appropriately adjust gas generation in response to weight of the occupant and in order not to actuate an airbag in a case where the occupant is a child. For example, disclosed in JP2003-83707A (hereinafter referred to as Patent reference 1) is a strain sensor, in which a bending stress applied to a strain detecting element does not change, thereby enhancing output accuracy.
The known strain sensor disclosed in Patent reference 1 is configured so that inner surfaces of a first upper washer contacting an upper surface of a sensor board and a first lower washer contacting a lower surface of the sensor board are press-fitted into a first cylinder which is press-fitted into a first fixing hole formed at the sensor board, and so that inner surfaces of a second upper washer contacting the upper surface of the sensor board and a second lower washer contacting the lower surface of the sensor board are press-fitted into a second cylinder, which is press-fitted into a second fixing hole formed at the sensor board, and so that inner surfaces of a detector upper washer contacting the upper surface of the sensor board and a detector lower washer contacting the lower surface of the sensor board are press-fitted into a detecting cylinder which is press-fitted into a detecting hole formed at the sensor board.
As the known strain sensor disclosed in Patent reference 1 is configured from the first washer and the second washer, which are separate and independent from each other, two pairs of washers may be press-fitted into the first and second fixing holes of the sensor board via the first and second cylinders, respectively. However, in a case where the first and second washers are integrally connected by means of an connecting portion on which an amplifier is mounted as described in, for example, Patent reference 1, two fixing portions (corresponding to the first and second washers) may not to be press-fitted into the respective holes in the manner disclosed in Patent reference 1 because of, for example, a dimensional difference between the two fixing portions and the fixing holes formed at the sensor board or because of a dimensional error. Further, if the two fixing portions are forced to be press-fitted into the corresponding fixing holes, an internal stress is generated at the sensor board and the load may not be accurately detected.
In such a case, as described in JP2009-128107A (hereinafter referred to as Patent reference 2), any one of the two fixing portions is press-fitted onto to a fixing shaft that is press-fitted into a strain generating body (a sensor substrate), and the other one of the two fixing portions is fitted onto the fixing shaft that is press-fitted in the strain generating body in a manner that a play is provided between the other one of the two fixing portion and the fixing shaft. Thus, the internal stress is restricted from occurring at the strain generating body.
By bringing the other one of the two fixing portions to be fitted onto the strain generating body in a manner that the play is provided therebetween, the internal stress is restricted from occurring at the strain generating body, however, a looseness between the other one of the two fixing portions and the strain generating body in a direction intersecting with a lengthwise direction of the strain generating body is not restricted, which may adversely affect enhancement of detection ability.
According to Patent reference 2, in a case where the load detection device is transported as a unit, a washer is press-fitted onto an end shaft portion of the other one of the fixing portions so that the other one of the fixing portions and the strain generating body are securely joined to each other. Due to the washer added thereto, the number of parts and weight of the load detection device increase. At the same time, an assembling work load increases, which may result in a new problem of a cost increase. Further, due to the washer added, a tightening axial force may become unstable, which may bring reduction in the load detection ability.
A need thus exists for a load detection device for a vehicle seat, which is not susceptible to the drawback mentioned above.